Solid substances are generally covered at the surface with an oxide film formed as the result of coupling between the atoms constituting solid substance and the oxygen in the atmospheric air. The introduction of impurity material to a solid substance has been conducted using an ion implantation or the like physical procedures, from above the oxide film which is kept on the surface as it is. Namely, irradiating the surface of a solid substance with ion of impurity source energized with electric fields, etc., thereby introducing the impurity into the inside.
As the result of recent development of finer device technologies in the device sector, it is requested to form a junction in a shallower profile. The low-energy ion implantation is a conventional technology used for forming a shallow junction. A number of improvements have been introduced to the low-energy ion implantation technology; for example, pulling ion out of the source using a substantially high voltage and then reducing the speed at a later stage. In this way, a low-energy implantation is realized while keeping the beam current value at a substantially high level. Such technologies have been successful in providing impurity layers in a profile as shallow as several 10 nm; and the layers have been in practical use in the semiconductor device industry.
Plasma doping technology is attracting the attention as a new technology for forming the junction in a still shallower profile. In the plasma doping process, plasma containing a certain desired particle is made to have contact with the surface of a target semiconductor substrate; thereby, the certain desired particle is introduced to the surface. Since the energy of plasma is as low as several hundreds volts at the highest, it is a suitable vehicle for forming an impurity layer in a shallow profile. According to experimental reports, the shallow junctions of ten-odd nm to several 10s nm deep have been formed. The Technical Digest of Symposium on VLSI Technology, Honolulu, p 110, (2000), discloses an experimental result; according to which, depth of a P-type junction is the smallest ever, or 7 nm.
Gaseous phase doping method which uses a gas source is also proposed in the IWJT, p. 19, (2000), the Japan Vacuum. Science Technology, A16, p 1, (1998), the Silicon Technology, No. 39, 18th Jun. 2002, and other publications. According to the method, a semiconductor substrate is heated in the normal pressure atmosphere of hydrogen, and B2H6, PH3 are supplied thereto for providing impurity diffusion layers, P-type and N-type. The hydrogen carrier gas is effective for removing the natural oxidation film sticking on the silicon surface, and for keeping the surface clean. So, it is advantageous in suppressing a surface segregation of the impurity, a.o. boron. For decomposing the gas, it requires a high temperature, generally higher than 600° C. The Silicon Technology, No. 39, 18 Jun. 2002, for example, reports an experimental result of forming a high-concentration shallow junction, in which a semiconductor substrate is heated to 900° C. and a 1 ppm B2H6 gas is delivered. Defining a level of boron concentration 1×1018/cm3 as the junction depth, depth of a junction attained in the above experiment is approximately the same as that reported in the earlier-described technology, 7 nm.
The above-described plasma doping and the low-energy ion implantation are the new technologies proposed to meeting the needs of a shallower junction, which needs have arisen as the result of efforts to implement finer device technologies. According to the recent publications, shallow junctions of ten-odd nm to several 10s nm deep have already been realized on experimental basis. The shallowest ever P-type junction is 7 nm, as already described in the above. In the face of continuous developments towards finer processing technologies, the needs of a new technology that can form an impurity layer through an easy process in a still shallower profile are growing. The plasma doping is a process for implanting the particles having acceleration energy into a semiconductor substrate, although the energy is small. So, it seems to be difficult for the plasma process to form an impurity layer in a still shallower profile. The gaseous phase doping which uses a gaseous source is a technology for forming an impurity layer taking advantage of the surface reaction by supplying a dopant having no acceleration energy to the substrate. This technology appears to be promising as a breakthrough to the limit in the conventional technologies which irradiate the substrate with energized ion. However, as described earlier, the technology requires a high temperature, higher than 600° C., for decomposing the gas. The high temperature makes it impossible to use a photo resist for the mask material. As the result, it becomes necessary to form a CVD-SiO2, etc., or to provide a patterning; which leads to an increase in the process steps for forming transistors.